Orbital angular momentum and beyond in free-space optical communications

Abstract

Orbital angular momentum (OAM), which describes tailoring the spatial physical dimension of light waves into a helical phase structure, has given rise to many applications in optical manipulation, microscopy, imaging, metrology, sensing, quantum science, and optical communications. Light beams carrying OAM feature two distinct characteristics, i.e., inherent orthogonality and unbounded states in principle, which are suitable for capacity scaling of optical communications. In this paper, we give an overview of OAM and beyond in free-space optical communications. The fundamentals of OAM, concept of optical communications using OAM, OAM modulation (OAM modulation based on spatial light modulator, high-speed OAM modulation, spatial array modulation), OAM multiplexing (spectrally efficient, high capacity, long distance), OAM multicasting (adaptive multicasting, N-dimensional multicasting), OAM communications in turbulence (adaptive optics, digital signal processing, auto-alignment system), structured light communications beyond OAM (Bessel beams, Airy beams, vector beams), diverse and robust communications using OAM and beyond (multiple scenes, turbulence-resilient communications, intelligent communications) are comprehensively reviewed. The prospects and challenges of optical communications using OAM and beyond are also discussed at the end. In the future, there will be more opportunities in exploiting extensive advanced applications from OAM beams to more general structured light.

Towards higher-dimensional structured light

“…Here the many DoFs and dimensionality of structured light come to the fore. The idea is to exploit the spatial degree of freedom of light, referred to as space-division multiplexing (SDM) 31 , 32 or its sister mode division multiplexing (MDM) 33 , for more channels and more capacity per channel, and has gained momentum in recent years (see refs., 10 – 12 , 34 – 37 for recent reviews). Topical among the multiplexing techniques is the use of OAM modes, particularly in conjunction with other DoFs 38 , as illustrated in Fig.…”

Self-healing of structured light: a review

“…14(a)) [178]. Such FSO links have been studied using different types of beams such as nonuniformly correlated Bessel beams (NUCBs) [179], Airy beams [180][181][182][183][184], OAM beams [185][186][187][188], and complex multi-vortex beams [189]. Since BG beams change from a Bessel structure to an annular ring, this is a disadvantage for long-range communication.…”

Correlations in Scattered Phase Singular Beams

“…These beams carry an orbital angular momentum (OAM) of mℏ per photon due to its azimuthal phase, where m is the order or topological charge defined as number of helices completed in one wavelength. The propagation of these light beams through various media such as turbid media [17,18], turbulence atmosphere [19,20], and under water [21,22] have attracted lot of interest in recent days for utilizing them for communication applications [23][24][25][26][27][28][29][30]. The vortices can be generated using computer generated holography [31,32] along with the help of spatial light modulator [33,34], spiral phase plate [35,36], and using an astigmatic mode converter [37].…”